Isotope exchange process



ISOTPE EXCHANGE PRCESS Richard W. Woodard, @ak Ridge, Tenn., assignor to .the United States of America as represented hy the United States Atomic Energy Cossion Application Jniy 26, 1949, Serial No. iii

claims. (ci. 24m- 1.5)

My invention relates to a method for changing the isotopic content of an ionic form of an element in solution, and to the utilization of this method for attaining isotopic enrichment with respect to one or more particular isotopes of the element.

it has previously been found that, in the ca/se of certain elements, isotopic exchange can take place in solution between ions of the element in different valence states. The conditions which make such exchange possible, however, are different for different elements, and no operative conditions have thus far been found for a great many of the elements. Even when such exchange has been found to take place, it has seldom been found possible to utilize the process for eecting isotopic enrichment when starting with the naturally occurring isotopic distribution. Such enrichment has been achieved in the case of a few elements of low atomic weight, but it has generally been believed that enrichment would be practically irnpossible for elements of atomic weight substantially above 40.

An object of my present invention is to eect isotopic exchange between two forms of uranium of dilerent valence states.

Another object of my invention is to provide a process for achieving isotopic exchange in solution, at a practical rate, between ionic species of uranium of diierent valence states.

A further object is to provide a process for isotopicallly enriching uranium with respect to one or more particular isotopes by means of isotopic exchange.

A still further object is to provide a semi-continuous multi-stage isotope exchange process for effecting enrichment of naturally occurring uranium with respect to one of its isotopes.

Additional objects and advantages of my invention will be apparent from the following description.

in accordance with my present invention, isotopic exchange between uranium ions is effected by contacting ions of tetravalent and hexavalen't uranium in an aqueous acidic solution of carefully controlled acidity and thereafter separating the ionic species. When `effecting the eX- change in this manner, have found that the equilibrium attained is not strictly a statistical `distribution `of the isotopic species, but that the lowel valence state ion is enriched with respect to the higher atomic `weight isotope, and vice versa. When three or more isotopic species are present, the highest atomic weight isotope tends to concentrate in the lower valence l-state and the remainder in the higher valence state. Contrary -.to expectations for an element of such high atomic weight, the 'single-stage enrichment factor thus attainable is sufficiently high .to make feasible substantially complete isolation of a single isotope by multi-stage operation.

The solutions in which isotopic exchange `is eifected in accordance with the present invention may -be aqueous solutions of any uranous and uranyl .salts which are known to be stable in solution at the .pH -values hereinafter discussed. From the standpoints of availability, ease of preparation for recycle, and convenience in adjusting the pH of the 'resulting so1utions,the salts of 'the 2,787,587 iiatented Apr. 2, 1957 strong mineral acids are preferred. Examples of such salts are uranyl chloride, uranous chloride, uranous oxychloride, uranyl perchlorate, uranous perchlorate, uranyl sulfate, uranous sulfate, and the iike. The nitrates are undesirable due to the instability of the uranous ion in nitric acid solution. However, such properties have iong been known, and one skilled in the art can readily choose suitable salts or acids for preparing solutions of adequate stability for the present process. The solutions employed, whether prepared by dissolving salts in water, by dissolving oxi'dcs in acid, by partially oxidizing uranous solutions, by partially reducing uranyl solutions, or by other equivalent processes, may all be termed acid solutions of uranous and uranyl ions and will be referred to as such in the appended claims.

I have found that control of the acidity of the solutions employed is essential for attaining practical rates of isotope exchange. Below a pH of 2, the rate constant of the exchange reaction decreases approximately linearly with pH, and at a pH substantially below l the rate is so slow as to be of practical utility. ln fact, the exchange reaction can be practically completely stopped before equilibrium is attained by strongly acidifying the solution.

It is unnecessary, for attaining practical exchange rates, to maintain the pH of the solution above 2.0. However, in view of the linear relationship between rate constant and pH it is obvious that for optimum rate the pH should be as high as possible without encountering interfering side elfects. The upper pH limit has been found to be that at which polymeric uranous 'ions are formed in the solution. For any particular solution, this upper limit will be 'dependent upon the temperature and the concentration of uranous ions in solution. At normal atmospheric temperatures and concentra-tions of uranous ions as low as l0-3 molar, or lower, the upper pH limit may be as high at 2.5, or even higher. However, for the preferred concentrations of uranous ions, of the order of 10-1 molar, and for moderately elevated temperatures, the upper pH limit will usually be in the neighborhood of 2.0. l'n any event, the upper limit 'for any particular solution may readily be determined by the appearance of polymeric uranous ions, with resulting changes in the absorption spectrum of the solution. Solutions containing uranyl ions and monomeric uranous ions are dark green in color. On the formation of polymeric uranous ions the solution darkens and becomes brown to almost black, depending upon the concentration. This change may be followed speotrographically, the molar extinction coeflicients at 400 millimicrons being approximately 7 for the uranyl ion, approximately 0 for monomeric uranous ions, and approximately for polymeric uranous ions.

Since both the isotope exchange reaction and the poly- Inerization reaction are (relatively slow und-cr the conditions of the present process, :the optimum pH will be that at which the .exchange :equilibrium is attained Shortly before the appearance .of the polymeric uranous ions. In most cases, however, it will be sufficient .to maintain the pH between ll.0 and 2.0, ibut preferably between 1.5 and 1.9.

While the mechanism of the exchange `reaction of the present invention has not been established, it has been observed that the optimum conditions lfor exchange `correlate with the optimum conditions for the formation of hydrolyzedmonomeric uranous ions .in solution. Thus, the ratio of UOH+3 to U+4 yin solutions of uranous chloride up to 10-2 molar, kat about 25 C., has been found to be represented by the following equation:

[UOlHH] Vproximately equal concentrations of both.

3 Similarly, for uranous perchlorate, the equation is:

[Uon+]` g 19g ;:.[U+] :1ipH 144 Thus, the conditions for satisfactory isotope exchange in accordance with the present invention are the presence of substantial concentrations of hydrolyzed monomeric uranous ions in solution and the absence of substantial concentrations of polymeric uranous ions.

The total concentration of uranium in the exchange solution should be as high as possible, to avoid the handling of 'large volumes of very dilute solutions. Solubility and polymerization considerations, however, limit the concentration of uranous ion for the present process to below about 0.5 molar. y The ratio of uranyl to uranous ions is not critical, but I generally prefer to employ ap- Suitable solutions are 0.05 to 0.5 molar with respect to both uranyl and uranous ions, but I prefer to employ solutions which are about 0.1 molar with respect to both ions.

The reactiontemperature for the isotope exchange may range from normal atmospheric temperatures, or below, to the normal boiling point of the solution, or even higher if the solution is maintained under p-ressure. Since the use of pressure vessels is inconvenient, it is generally preferred to operate within a temperature range of about .2S-75 C. High temperatures will increase the exchange reaction rate, but may adversely aiect the enrichment equilibrium. For simple exchange reactions I therefore prefer to opera-te at elevated temperatures in order to attain a rapid reaction rate, but for enrichment purposes I prefer to operate at about 2530 C., or even below, in order to attain a favorable enrichment factor.

The isotope exchange of the presen-t invention is photosensitive, and the presence of light considerably accelerates the reaction. Although useful reaction rates are attainable when effecting the exchange in the dark, I prefer to carry out the reaction in -the light. Any source of visible light, e. g. daylight or tungsten filament incandescent light, is suitable for this purpose. Ultraviolet light does not appear to increase the reaction rate substantially, when used in addition to visible light, but may be employed if desired.

The reaction time for attaining equilibrium lin the exchange reaction will, of course, `depend on the factors such as pH, temperature, and light, which aiect thereaction rate. When employing the preferred reaction conditions, as discussed above, Isuitable reaction times are from 45 minutes to 120 minutes, with a reaction time of about one hour generally being preferred. In any event the time required to reach equilibrium in the exchange can be determined for any chosen reaction conditions by periodically sampling the solution and ldetermining the isotopic content of the uranous uranium and Vof Vthe uranyl uranium in the sample.

In carrying out a single stage exchange in accordance with the present invention the exchange solution is prepared in accordance with the above description, using sources of uranyl and uranous ions of either the same or 'dilerent isotopic constitution, as desired. This solution is then maintained at the chosen pH and temperature, preferably in the light, until the exchange reaction is substantially complete. It is generally preferable'at this point to acidify the solution, e. g. to about 3N.,'to prevent any possible adverse exchange reaction during the subsequent chemical processing. The uranyl and uranous components of the solution may then be separated by conventional methods such as selective precipitationV of the uranium of one Valence state, or selective extraction of the uranium of one valence `state by means of organic solvents using selective chelating agents or the like. Many suitable separation processes are well known in the art, and this step of the present process is not claimed as novel herein.

In a multi-stage process, the separated uranyl uranium from the preceding stage may be used as the source of both the uranyl and uranous uranium for the succeeding enrichment stage (enrichment as to the lighter isotope); and the uranous uranium from the preceding stage may be used as the source of both the uranyl and uranous uranium for the succeeding depletion stage (depletion as to the lighter isotope). Preferably, however, a multistage enrichment process is carried out by a batchcountercnrrent procedure which Vdoes not require oxidation or reduction between stages. An example of one stage of such a process is illustrated in the ow diagram constituting the accompanying drawing, which is selfexplanatory. In such a process, the exchage solution for any intermediate stage, for enriching with respect to the lighter isotope, isprepared from the uranyl uranium separated from the preceding stage and from the uranous uranium separated from the succeeding stage.

In any such multi-stage operation, the chemical processing required to convert separated uranium from one stage into suitable feed solution for an adjacent stage may be accomplished by processes well known in the art, and such conversion steps are not claimed as novel herein.

The following example illustrates the exchange of isotopes between uranyl and uranous ion-s of dilerent initial isotopic constitution, utilizing the differences in radioactivity to follow the exchange reaction:

EXAMPLE I A stock solution was prepared from almost pure Um, in the form of UCl4, in a concentration of 6.25 grams U per liter. This solution had a specilic activity of 25,000 counts per minute per milliliter.

A second stock solution was prepared from uranium enhanced in U234 and U235, in the form of UOzClz, alsov in a concentration of 6.25 grams U per liter. This solution had a specific activity of 132,500 counts per minute per milliliter.

Systems were made up by taking 2.0 ml. of each stock solution, adding suflicient hydrochloric acid to give the desired pH, and diluting to a volume of 20.0 ml. The pH of each such exchange solution was then measured potentiometrically, and a 2.0 ml. Valiquot was withdrawn to determine actual initial content of U+1 Vand Ul6 (because of partial oxidation of the Ut4 stock solution on storage). Each of the exchange solutions was maintained at 28 C., in the light, and 2.0 ml. aliquots were withdrawn periodically for analysis. The U+4 content of each aliquot was separated` by a conventional cupferron-chloroform extraction. The chloroform solution of the uranium cupferride was diluted to 10.0 ml. from which a 2.0 ml. aliquot was taken for radioactivity determination by conventional counting methods, and a second 2.0 ml. aliquot was taken for determination of the uranium content byV conventional colorimetric analysis.

The exchange reaction, effected as above described, maybe represented as:

A=mierograms U as U*02+2 at t=0 R=W at time=t Quantities A, B, and t are all known, and R is determined by solving simultaneous equations involving the weight and radioactivity of the tetravalent uranium inthe system at time=t.

The results of the exchange reactions carried out as described above are shown in Table 1, together with the ment with respect to U235). This was accomplished by conventional procedures, i. e. precipitating the uranyl ion as ammonium diur'anate, dissolving the precipitate in aque` ous nitric acid, reprecipitating with hydrogen peroxide,

calculated rate constant for each `pH zand `reaction time and lcalcining the lat-ter precipitate to produce U03. The

N own-A and B in Table 1 are actually only one-nitieth of the amount of tetravalent and uranyl ion present ln the original system. These values are employed since the activity and colorlmetric determinations also represent one-fiftieth of the original system.

The exchange reaction of the above example has also was chlorinated with hexachloropropene to form UCl4,

been successfully effected using U233 initially in the hexavalent state, using U233 initially in the tetravalent state, and using U02SO4 and U(S04)2 in place of U02Clz and UCllr.

The following example illustrates the utilization of the exchange reaction .of the present invention in a multistage process for the isotopic enrichment of natural uranium:

EXAMPLE II A quantity of uranium oxide (U03) of known isotopic concentration. (normal at the start of a run) was divided into two equal portions. One of these was chlorinated to UCLr with hexachloropropene, the other was prepared as U02Cl2,XH20 by dissolving U03 in hydrochloric acid and evaporating to dryness on the hotplate. The two salts, UClt and U02C12, were then dissolved in a sutlicient quantity of `Water to give a 4% uranium solution, i. e. 2% with respect to U+4 and v2% U as U02+2. The pH of the solution was checked at this point and usually it was 1.54.6, which is within the preferred range forexchange. Suicient time, usually about one hour, was allowed for the system to .come to equilibrium at 2830 C. The exchange reaction was stopped at this point by the addition of a Sueient quantity of hydrochloric acid to raise the normality to 3N. A five percent excess over the theoretical amount of oxalie acid necessary to precipitate all U+4 ion was added and the solution stirred to aid in the formation et an easily filtered uranous oxalate precipitate. The oxalate precipitate was washed several times with dilute. HC1 containing oxalic acid, combining these washings with the original lti te,

The filtrate from the oxalate precipitation was utilized to prepare the feed for the next enrichment stag@ (enrichand the other of which was dissolved in aqueous hydrochloric acid and evaporated to dryness to produce U0zCl2.XH20. The uranous and uranyl salts thus obtained were then dissolved in water, each at a concentration of 2% U, to form the exchange solution for the succeeding enrichment stage.

The above procedure was repeated, with respect to the oxalate filtrate, for seven more stages, to make a total of nine enrichment stages, each stage involving a volume reduction. y

The uranous oxalate 'precipitate from the initial stage, as described above, was utilized to prepare the feed yfor the rst of a series of depletion stages (depletion with respect to Um). This, again, was accomplished by well known methods, i. e., igniting the oxalate to produce U30s, dissolving the U30s in nitric acid, precipitating the uranium from the nitric acid solution with hydrogen peroxide, and calcining the latter precipitate to produce U03. The U03 was then divided into two equal parts, one of which was chlorinated with hexachloropropene to produce UCl4, and the other of which was dissolved in aqueous hydrochloric acid and evaporated to dryness to produce U02Cl2JU-I2O. The resulting uranous and uranyl salts were then dissolved in water, each at a concentration of 2% U, to form the exchange solution for the succeeding depletion stage.

This procedure was then repeated, with respect to the oxalate precipitate, for eight more stages, to make a total of nine depetion stages, each stage involving a 50% volume reduction.

A summary or" the above enrichment runs is given in Table 2, and a summary of the depletion runs in Table 3. In each case initial, intermediate, and iinal assays for U235 are included.

Table 2 ENRICHMENT RUNS Total U in grams Total Time of M l. Conc. Oxalic Assayof Stage Volume o pH Stand HC1 Add. Acid Add Samples, System (Mln.) at end of to PPT Percent As UH As U02 (M1.) Exchange U+4 (g.) U2

396 399 40, 000 l. 198 198 20, 000 l, 68 96 96 9, 600 1. 53 4T 46 4, 600 l. 55 23 22. 5 2, 200 1. 55 10. S 10. 8 1, 080 1. 58 5.. 5 5. 2 700 1. 40 3. 5 3. 5 350 1. 49 1. 6 1. 6 160 1. 52

affamati A stage separation factor of 1.0012 may be calculated from the assays of either the enrichment runs or the depletion runs of the above example. From this it may be seen that by greatly increasingv the number of stages U335 may be substantially completely separated from natural uranium by the process of the present invention. Obviously, however, for such a greatly increased number of stages a 50% volume reduction per stage would be very undesirable. This may be avoided by employing a counter-current batch procedure such as hereinbefore referred to and illustrated, as to one stage, in the accompanying ow diagram. After a countercurrent cascade of this type has reached operating equilibrium, the uranyl uranium may be passed stagewise up the cascade, and the uranous uranium stagewise down the cascade Without the necessity for -a substantial volume change from stage to stage.

It will be apparent that the procedure of Example VII may be utilized for the separation of isotopes other than U235. For'example, natural uranium which has been depleted of U235 by the electromagnetic separation process, and subsequently stored for a long period of time, contains substantial quantitiesof U234 in addition to the otherwise substantially pure U238. These two isotopes, both of which have recognized utility, obviously can be successfully separated by the enrichment process illustrated in connection with the lseparation of U235.

In general, it may be said that the above examples are merely illustrative and should not be construed as limiting the scope of my invention. Numerous other isotope combinations may be separated, various other exchange reaction conditions may be employed, and many other interstage chemical conversion steps may be utilized, within the scope of the foregoing description. The scope of my invention should be understood to be limited only as indicated by the appended claims.

I claim:

l. A process for effecting isotope exchange between uranyl ions and uranous ions which comprises digesting an aqueous acidic solution of uranyl ions and uranous ions at a pH of 1.0-2.0.

2. Aprocess forv effecting isotope exchange between uranyl ions and uranous ions which comprises digesting an aqueous mineral acid solution of uranyly ions and uranous ions at a temperature of 25-75" C. andinthe presence of light, while maintaining the pH of said solution between 1.5 and 1.9.. l

3. A lprocess for effecting isotope 'exchangebetween uranyl ions and uranous ions which comprises digestingV an aqueous acidic solutionof uranyl ions and. uranous n ence of light, and thereafter separating the uranyl and ions at a pH above '1.0 and below the -pH at which polymeric uranous ions are formed in said solution.'

I 4. A process for effecting isotope `exchange betweenv uranyl ions vand uranous ions which comprises digesting@ uranyl and uranous components of 4said solution. 5. A process for effecting isotope exchange between uranous components of said solution'.l

6. A process for isotopically enriching, with respect to `a lighter isotope, uranium initially containing a plurality of isotopes, which comprises forming from said uranium an aqueous acidic solution of uranyl and uranous ions, digesting said solution at a pH of 1.0-2.0, separating the uranyl and uranous components of the digested solution, forming from the separated isotopically enriched uranyl componentpa fresh aqueous acidic solution of uranyl and uranous ions, and repeating said digesting and separating steps, l

7. The process of claim 6 in which the initial plurality of isotopes comprises essentially U235 and U23".

8. The process of claim 6 in which the initial plurality of isotopes consists essentiallyof U234 and U23.

9."A process for isotopically enriching, with respect to a lighter isotope, uranium initially containing a plurality of isotopes, which comprises forming from said uranium.

an aqueous mineral acid solution about 0.1 molar with respect to uranyl ions and about 0.1 molar with respect to uranous ions, digesting said solution for about one hour at a temperature of 25-75 C. and in the presence of light, while maintaining the pH of said solution between 1.5 and 1.9, separating the uranyl and uranous components of the digested solution, forming from the separated isotopically enriched uranyl component a fresh mineral acid solution of uranyl and uranous ions, and repeating said digesting and separating steps.

110. A process for isotopically enriching and depleting, with respect to a'lighter isotope, uranium initially containing a plurality of isotopes, which comprises forming from said uranium an aqueous mineral acid solution of uranyl and monomeric hydrolyzed uranous ions, digesting said solution' to eiect isotope exchange, separating the uranyl and uranous components of the digested solution, utilizing the separated isotopically enriched uranyl component as the source of uranyl ions for a fresh aqueous mineral acidsolutionof-uranyl Vions vand monomeric hydrolyzed uranous ions, and repeating said digesting and separating steps; utilizing the separated isotopically depleted uranous component from said first digesting step as the source of uranous ions for a fresh aqueous mineral acid solution ofuranyl ions and monomeric hydrolyzed uranous ions, and repeating said digesting and separating steps.

11. The process of claim 10 in which the initial plul rality of isotopes comprises essentially U235 and U233- wams? 0.1 molar with respect to uranons ions, digesting said solution for about one hour at a temperature of 25-30 C. in the presence of light while maintaining the pH of said solution between 1.5 and 1.9, separating the uranyl and uranous components of the digested solution, utilizing the separated isotopically enriched uranyl component as the source of uranyl ions for a fresh hydrochloric acid solution of uranyl ions and uranous ions, and repeating said digesting and separating steps; utilizing the separated isotopically depleted uranous component from said first digesting step as the source of uranous ions for a fresh 10 aqueous hydrochloric acid solution of uranyl and uranous ions, and repeating said digesting and separating steps.

14. The process of claim 13 in which the initial plurality of isotopes comprises essentially U235 and U2.

15. The process of claim 13 in which the initial plurality of isotopes consists essentially of U234 and U23".

References Cited in the file of this patent Friend: Textbook of Inorganic Chemistry, vol. VII,

10 part III, p. 288; published in 1926 by Charles Grin and Co., Ltd., London. (Copy in Scientific Library.) 

4. A PROCESS FOR EFFECTING ISOTOPE EXCHANGE BETWEEN URANYL IONS AND URANOUS IONS WHICH COMPRISES DIGESTING AN AQUEOUS ACIDIC SOLUTION OF URANYL IONS AND MONOMERIC HYDROLYZED URANOUS IONS AT A TEMPERATURE OF 25-75*C. AND IN THE PRESENCE OF LIGHT AND THEREAFTER SEPARATING THE URANYL AND URANOUS COMPONENTS OF SAID SOLUTION. 